1. Field of the Invention
The present invention relates to the field of printing by expelling ink from a print head onto a print medium. More particularly, the present invention relates to piezoelectrically driven ink printheads.
2. Description of Related Art
In the printing arts, ink printheads of various configurations are publically available. Typically, an ink printhead has apertures or nozzles from which ink droplets are expelled onto a print medium, and the ink is routed internally through the printhead. Piezoelectric transducers can be used to generate pressure necessary to expel droplets of ink from the nozzles of the printhead. In situations where a printhead includes multiple nozzles, it is often desirable to be able to selectively expel ink droplets from a specific nozzle and not the other nozzles. Conventional solutions known in the art, isolate the nozzles from each other by long narrow passages that damp pressure surges in the ink fluid provided to the nozzles from a common source. Heaters can also be located at each nozzle, for the purpose of reducing ink viscosity at a specific nozzle. Thus, when a droplet is to be ejected from a specific nozzle, the heater at that nozzle is activated to heat ink at the nozzle so that when a pressure pulse is applied to the ink fluid, the ink viscosity at the nozzle is reduced enough so that a droplet of ink will be expelled from the nozzle, while the higher viscosity of the (colder) ink at the other nozzles remains high enough to prevent ejection of ink droplets from those other nozzles.
However, such conventional designs suffer from various disadvantages. For example, long, narrow passageways for transmitting ink to a particular nozzle are more likely to become clogged, especially when inks with pigment particles are used, or when metallic inks, whose components tend to separate, are used. In addition, when a clog does occur, the length and narrowness of the passageways increases the difficulty of successfully cleaning the printhead and clearing the passageways of blockages. The physical structure of such printheads is also somewhat complicated, and therefore such printheads tend to be more expensive to manufacture.
Accordingly, a need exists for a printhead that is reliable, easy to clean, simple in design, and economical to manufacture.
In accordance with exemplary embodiments of the present invention, a printhead is provided with a large ink chamber. One side of the ink chamber is formed by a membrane. Nozzles for expelling ink from the printhead are formed by apertures in the membrane, and piezoelectric actuators (hereafter xe2x80x9cpiezoxe2x80x9d) are provided near each nozzle aperture, on the membrane. The piezos expel ink from a nozzle aperture by flexing the membrane at the aperture to create a pressure pulse in the ink fluid near the aperture, that is sufficient to expel a droplet of ink fluid through the nozzle aperture. Ribs are also provided on the membrane, to separate or isolate adjacent nozzle apertures. In accordance with an exemplary embodiment of the invention, the ribs isolate a nozzle aperture in the membrane from the other nozzle apertures in the membrane by supporting a boundary of a membrane segment surrounding the isolated nozzle aperture. Thus, when the piezos on the membrane segment of the isolated nozzle aperture are actuated to flex the membrane segment of the nozzle aperture, the ribs act as stiffeners to bound the flexing so that the actuated piezos do not also flex other parts of the membrane beyond the membrane segment of the nozzle aperture.
In accordance with another exemplary embodiment of the invention, the ribs protrude slightly into the common ink chamber of the printhead. Thus when a membrane segment of a nozzle aperture is flexed by the piezos attached to it, the resulting pressure pulse in the ink fluid of the common chamber will tend to be deflected upwards or away from the membrane segment. This is because the ribs surrounding or bounding the membrane segment, act as a fence to deflect or inhibit the pressure pulse from traveling laterally along the membrane through the ink fluid to adjacent nozzle apertures.
These effects, along with the natural damping effects of the ink fluid and dissipation of the pressure pulse as it expands away from its origin, prevent ink droplets from being expelled from nozzle apertures in the membrane whose piezos have not been activated.
In accordance with other exemplary embodiments of the invention, the piezos can be provided either on the outside of the membrane, or on the inside of the membrane immersed in the ink fluid inside the common ink chamber. Stiffening ribs bounding or defining nozzle aperture membrane segments can also be provided on the outer surface of the membrane, in addition to or instead of, ribs provided on an interior surface of the membrane inside the common ink chamber. In accordance with another exemplary embodiment of the invention, ribs can be provided on an inside surface of the membrane, within the common ink chamber, in a configuration that impedes lateral travel of a pressure pulse in the ink fluid along the surface of the membrane, without increasing local rigidity of the membrane. The ribs can be formed using a material different from the membrane, or can be formed using the same material. The ribs can be formed by providing the membrane with additional thicknesses at appropriate locations. The ribs can also be formed by corrugating the membrane at appropriate locations.